Excited in Air at the Foot of Waterfalls. 425 



will be noticed that the greatest amount of radioactivity 

 observed at Niagara Falls was 0*3, while the least observed 

 in Toronto during the period covered by the experiments was 

 0"6. The numbers also show that on the average the radio- 

 activity excited at Toronto was at least from six to seven 

 times greater than that induced at the Falls. 



The observations were not made in the two places at the 

 same time, and a direct comparison is therefore impossible, 

 but as there was no break in the weather during the progress 

 of the experiments, and as the values found in Toronto before 

 going to the Falls were almost the same as those obtained on 

 returning, one is warranted in concluding that the air at the 

 foot of the Falls permanently possesses less power to excite 

 radioactivity than the air of localities at some distance. 



While the experiments were in progress the spray was 

 frequently examined for radioactivity. This was done by 

 collecting a quantity, evaporating it, and testing the con- 

 taining vessel. In no case was any trace of radioactivity 

 observed. 



Assuming then that excited radioactivity is caused by the 

 presence in the atmosphere of some peculiar constituent, the 

 experiments which have been described would seem to show 

 that this constituent is present to a much less degree in the 

 atmosphere in the neighbourhood of waterfalls than in places 

 remote from them. 



That it is possible to reduce the amount of this constituent 

 present in the atmosphere is shown by some experiments 

 recently performed in the Physical Laboratory at Toronto. 

 The windows and doors of a large room, which had been well 

 aired, were closed and, while of course they were not made 

 air-tight, it may be assumed, as they were close-fitting, that 

 the quantity of outside air which they admitted was relatively 

 small. An electrical machine at one end of the room was set 

 in action and a number of zinc disks attached to a wire lead- 

 ing from its negative terminal were suspended for an hour 

 simultaneously at different points in the room. Upon testing 

 these disks for radioactivity by the method employed by 

 C. T. R. Wilson* it was found that radioactivity excited in 

 the disks regularly increased with the distance from the 

 machine. 



It was also found, when a series of disks were consecutively 

 exposed for an hour in a fixed position in the room after it 

 had been well-aired and closed, that the radioactivity excited 

 regularly diminished with the time. The admission of a 

 considerable volume of fresh air, as by the opening of a 

 * 0. T. K. Wilson, Proc. Roy. Soc, vol, Ixviii. p. 154. 



